The invention relates to a coupling device (switch) for a local network operating in the asynchronous transfer mode (ATM), which comprises a central cell memory (CCM) for buffer storage of packets (frames) formed by individual ATM cells and arriving on feed lines, a central memory control device for controlling the storage in the central cell memory (CCM) and the writing and reading processes of the packets into and from the central cell memory (CCM), a demultiplexer controlled by the central memory control device for transmitting the packets (frames) further on receiver lines. The control device further includes counting means for counting the ATM cells arriving in the central cell memory (CCM) and discarding means for discarding ATM cells packet by packet for preventing congestion in the central cell memory (CCM).
Blocks of fixed length are formed from the information (messages) to be transmitted in local networks which operate in the asynchronous transfer mode (ATM). Each block is supplemented with additional control information so as to form a transmission unit which is referred to as “ATM cell”. Each ATM cell is then composed of a cell header with a length of 5 bytes and an information field (payload) with a length of 48 bytes in which the information to be transmitted is contained.
The available transmission capacity (bandwidth) of a wide-band channel is subdivided into so-called virtual paths (VP) in ATM networks for different transmission purposes such as data transmission services, speech transmission services, video services (for example, video conferences or video databanks), or the transmission of multimedia information where speech, data, and video information are combined with one another. A virtual path in its turn comprises a group of virtual channels (VC). When a virtual link is made between a transmitter and a receiver, the suitable virtual path is first selected. Then part of the bandwidth is occupied as a virtual channel of this path. The VCI mark (virtual channel identifier) is used for the identification of the virtual channel, while the VPI mark (virtual path identifier) is used for the identification of the paths. The specification of VPI and VCI accordingly represents the logic identification for a single link.
When an ATM link is realized over a physical line, this ATM link has the full transmission rate of the physical line at its disposal. Since there is no mutual arrangement between transmission rates within physical lines and the individual ATM links, the traffic of an ATM link must be defined by means of certain parameters. This definition is necessary on the one hand to avoid overload situations in the network and on the other hand to safeguard a certain quality of ATM links.
To achieve this, a traffic contract between an ATM user and the network must be closed as regards the desired ATM link. The ATM network decides on the accepting of a new link on the basis of given traffic parameters, so that the traffic contract is not created until an ATM link is accepted. The ATM network thus ensures that the quality of the link is maintained as required for the ATM application provided this application complies with traffic parameter values agreed in the traffic contract. The UPC function (user parameter control) serves to monitor the traffic parameters.
Certain ATM adaptation layers (AAL) are defined depending on the supported ATM application. The arrangements of the useful data, i.e. larger data packets or synchronous bit streams, are realized in the ATM cells of 53 bytes length each time in the AAL. Several versions exist here:
AAL 1
This version realizes a synchronous bit stream and is used for the adaptation of synchronous speech or video channels, but not for the packet-oriented data communication.
AAL 3/4
This is the adaptation layer originally defined by the CCITT for packet-oriented data communication which comprises additional control structures for multiplexing different data packets on a common virtual channel. This service type supports both non-channel-oriented and VC-oriented LAN communication in ATM networks.
AAL 5
This is the standard for data communication in local ATM networks which is generally accepted at the moment, AAL 5 being much more simply constructed than AAL 3/4. Up to 48 bytes can be transported per ATM cell as a payload in AAL 5, against 44 in AAL 3/4, so that the relative quantity of control information is reduced.
In a transmission file of, for example, 216 bits, the information can be transmitted by means of approximately 1300 ATM cells in AAL 5, with the so-called EOM (end of message) bit which signals the end of the transmission being present in the header of the final cell.
A system of the kind mentioned in the opening paragraph is known from “Dynamics of TCP Traffic over ATM Networks”, Romanow, Floyd et al., IEEE-JSAC, vol. 13, No. 4, May 1995, pp. 633-644. This system relates to TCP (transmission control protocol) links in ATM networks, where it is the object to avoid congestion in the central memory in the coupling device. This system then is based on a congestion control for the central memory in accordance with the so-called frame discard method. The incoming frames are buffered in the central memory in a VC-oriented manner, and the entire frame belonging to a certain VC is discarded in the case in which congestion arises at the output of the coupling device.
This known system operates in particular by the so-called early packet discard method, which runs as follows: if there is a risk of a central cell memory becoming congested and at least one cell of a connection is identified as being defective, the entire packet is discarded the moment only a single cell is defective. The discarding process is such that those cells of a packet which have yet to be accepted are no longer stored. The packet to be discarded thus does not occupy any memory space. The number of data retransmissions is limited to the relevant discarded packet each time.
In the known system, the so-called resource management application ascertains the degree of filling of the buffer memory used and decides which packet is to be discarded. The effect which this has on the filling is further monitored and, if necessary, further packets are discarded. The discarding process of packets of various links is continued as long as the degree of filling still reaches up to a given threshold value. This known method is suitable for memories having a very great memory capacity, for example capacities of more than 10,000 cells. A disadvantage of this system, however, is that the VC-oriented arrangement of the memory requires an organizer which is specially designed for this arrangement. Since the adaptation process as to the degree of filling of the memory always takes a few milliseconds, a delay effect will arise which renders the use of the early packet drop method unsuitable for small (decentralized) buffer memories for cells.